engine misses at around 3k rpms under load.
fresh tune-up w/wires, new f/filter (was clogged), new fuel injectors, good fuel pump pressure @ fuel rail, checks ok on vacuum gauge (no leaks), no engine light on. just recently passed DEQ. bought car a couple years ago with the miss. nothing I’ve done seemed to fix the problem.
What mileage, what transmission? What modifications have been done to the car?
How are you determining it is a ‘miss’ ? A misfire, I assume. What do you feel when this happens?
first of all, thanks for responding. m/trans, 166k miles, all stock. I did get a miss fire code po301 a couple weeks ago when it started to miss fire going uphill in 4th gear, would not pull any more rpms, when I shifted into 5th gear (rpms dropped to about 2600) it then pulled enough power to get up the hill. this all happened at about 50-55 mph. it doesn’t feel like its fuel related, especially when it has enough fuel pump pressure 35-45 psi, and a new fuel filter, and no vacuum leaks (judging what the vacuum gauge reading 17-19", and 4 new fuel injectors. it’s strange to me that the engine code po301 only happened the one time when it has this problem in any gear, but under a heavier load it gets worse.
Based on that info, I would suggest running a wet and dry compression test on the engine. Your vacuum readings are a bit low, the intermittant miss and 301 code may mean the piston ring seal or valve seat seal is no longer 100%. With 166k on the engine, it may be worn out.
I assume it has new spark plugs… what about the coil?
A bad crankshaft position sensor can cause a random misfire.
But not cause the Check Engine light to turn on.
https://www.1aauto.com/content/articles/bad-or-failing-crankshaft-position-sensor-symptoms
Tester
thanks, that will be easy to check the voltage on that. it’s so easy to get to it.
Then just change it, they are cheap and the poor thing is old enough to order a beer in a bar!
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Love it! And a beer for you too. Thanks again.
A censored press is a tool for a corrupt minority.
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That 17 on the vacuum gauge could be considered a bit low so I might ask what the altitude is where you live. Just wondering whether or not the engine may be dropping compression on a cylinder.
Does it fluctuate that much while driving up hill? Sounds like the fuel pump is weak, the spec. is 43 PSI.
the factory reads 35 psi w/vacuum attached (fuel pressure reg.) and 45 off. checks out on both.
the heavier the engine has to pull is when it starts to act up, in fact today, when driving on a flat surface in 5th gear. as soon as the rpms reached 3k (70 mph) it started to act up. I’m starting to wonder if it couldn’t be the TPS. I’ll have to check the voltage there too.
and I have to say that the vacuum actually is 19-20 inches so I’m somewhat comfortable with those #'s.
I really appreciate the suggestions from everyone. I like this forum, first time user. I could tell that the previous owner was chasing the same problem. I think that whatever the problem is, it’s not the ordinary culprits. in fact, I believe it will be something simple in the end.
The more you post, the more it sound like weak spark. Check your long and short term fuel trims to make sure the engine isn’t going lean under the loads that make it miss but…
A weak spark will cause what you are experiencing. That means replace the coil or … maybe try decreasing the spark plug gap a small amount. If 0.46 in is called for, try 0.44 in.
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That 19 to 20 inches of vacuum is fine if the gauge needle is rock steady. If the needle is fluctuating then there is a problem.
Another possibility I might mention. Maybe there is an issue with the MAF sensor OR there is an intake tract leak between the throttle body and air filter.
Some years ago my Lincoln developed a buck at 45 to 50 MPH and which was not there all the time. The cause was a razor type cut in the PCV hose. Depending upon acceleration the hose would pinch down a bit and open the cut up a little which of course allowed some air to bypass the MAF. Replaced the hose and all was fine.
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